14.13.1 InnoDB Disk I/O

InnoDB uses asynchronous disk I/O where
possible, by creating a number of threads to handle I/O
operations, while permitting other database operations to proceed
while the I/O is still in progress. On Linux and Windows
platforms, InnoDB uses the available OS and library functions to
perform “native” asynchronous I/O. On other
platforms, InnoDB still uses I/O threads, but the threads may
actually wait for I/O requests to complete; this technique is
known as “simulated” asynchronous I/O.

Read-Ahead

If InnoDB can determine there is a high probability that data
might be needed soon, it performs read-ahead operations to bring
that data into the buffer pool so that it is available in memory.
Making a few large read requests for contiguous data can be more
efficient than making several small, spread-out requests. There
are two read-ahead heuristics in InnoDB:

In sequential read-ahead, if InnoDB notices
that the access pattern to a segment in the tablespace is
sequential, it posts in advance a batch of reads of database
pages to the I/O system.

In random read-ahead, if InnoDB notices
that some area in a tablespace seems to be in the process of
being fully read into the buffer pool, it posts the remaining
reads to the I/O system.

Doublewrite Buffer

InnoDB uses a novel file flush technique
involving a structure called the
doublewrite buffer,
which is enabled by default
(innodb_doublewrite=ON). It adds
safety to recovery following a crash or power outage, and improves
performance on most varieties of Unix by reducing the need for
fsync() operations.

Before writing pages to a data file, InnoDB
first writes them to a contiguous tablespace area called the
doublewrite buffer. Only after the write and the flush to the
doublewrite buffer has completed does InnoDB
write the pages to their proper positions in the data file. If
there is an operating system, storage subsystem, or
mysqld process crash in the middle of a page
write (causing a torn page
condition), InnoDB can later find a good copy
of the page from the doublewrite buffer during recovery.